The present invention relates to a close-up focusing device for photographic viewfinder cameras. It particularly relates to a close-up focusing device of the type wherein a supplementary lens is interposed in the optical field of the camera objective lens.
In order to increase the close-up focusing range of photographic cameras not equipped with interchangeable objective lenses of greater focal length, it is common to interpose a supplementary lens with the desired refractive power in the optical field of the objective lens. The use of the supplementary lens alters the effective focal length of the objective lens. Since the distance that the objective lens must be moved for focusing is in part determined by the focal length of the supplementary lens, the movement of the objective lens for a given change of focus distance is thus also altered. As a result, the focus adjustment mechanism, such as a rangefinder and viewfinder, must be compensated. With objective lenses of fixed foci, the object distance is determined by the focal length of the supplementary lens, requiring correction of the focus adjustment mechanism. With an objective lens of adjustable focus, the object distance for each setting of the objective must be corrected, conventionally by employing a table giving the actual object distance for each objective setting. In either situation, adjustment of the camera to the correct object distance is laborious. Moreover, the altered viewfinder parallax must also be corrected.
It has therefore been proposed to connect the supplementary lens with a table-like four legged stand, said stand being dimensioned such that the length of the legs correspond to the object distance and that the four legs of the stand indicate the image area. According to another proposal, a telescopic rod is connected for the same purpose with the mount of the supplementary lens, with a frame outlining the image area provided at the free end of the telescopic rod. Such devices make it possible to set the object distance accurately. Accurate parallax markings are also provided to correct the altered parallax. However, these devices have to be carried along, and further, such arrangements are unsuitable for numerous purposes, especially when contact with the object interferes with their use or precludes their use altogether.